In recent years, the reduction in the carbon dioxide emission has been ardently desired in order to deal with global warming. In addition, it is expected that the fossil fuel such as oil, coal, and natural gas is depleted in the near future and thus it is an urgent matter to secure earth-friendly energy resources to replace these. Hence, the development of power generation technology using sunlight, wind power, geothermal energy, nuclear power, or the like has been extensively conducted, and among them, the photovoltaic power generation has received particular attention in terms of high safety.
In the photovoltaic power generation, light energy is directly converted into electric power by using a photoelectric conversion element utilizing the photovoltaic effect. In general, the photoelectric conversion element has a structure consisting of a photoelectric conversion layer (light absorbing layer) sandwiched between a pair of electrodes, and light energy is converted into electrical energy in the photoelectric conversion layer. The photoelectric conversion element is formed into various types depending on the materials used in the photoelectric conversion layer or the form of the element, and a silicon-based photoelectric conversion element using single crystal Si, polycrystal Si or amorphous Si, a compound-based photoelectric conversion element using a compound semiconductor such as GaAs or CICS (a semiconductor consisting of copper (Cu), indium (In), gallium (Ga), and selenium (Se)), a dye-sensitized type photoelectric conversion element (Gratzel cell), or the like has been proposed and practically used.
Normally, solar cells using these photoelectric conversion elements are disposed on a roof or land. In recent years, however, semi-transparent type (see-through type) solar cells such as windows have attracted attention. These semi-transparent solar cells are not only simply able to generate electric power using sunlight irradiated to the window but are also able to reduce the penetration of solar heat (heat rays) into the room by solar radiation. Hence, it is expected not only the power generation effect but also the effect of air-conditioning cost reduction.
Such a solar cell which is a semi-transparent type capable of being installed to a window and also can be installed to the existing building by simply pasting is expected to be a new field to grow from now on as a building-integrated photovoltaic (BIPV) system.
Both the two electrodes constituting the solar cell (organic photoelectric conversion element) are required to be transparent in order to achieve the semi-transparent solar cell. A first electrode formed on the substrate is generally a transparent electrode composed of a metal oxide material such as ITO, but a second electrode (counter electrode) is conventionally a non-transparent electrode composed of a metallic material in many cases and thus improvement is desired.
As an example using a transparent electrode as the two electrodes constituting a photoelectric conversion element, for example, a method of using a transparent electrode composed of ITO in both the first electrode and the second electrode (Patent Literature 1), a method of using a conductive polymer in two electrodes (Non-Patent Literature 1), a method combining a conductive polymer and a metal mesh electrode (Patent Literature 2), a method of using a metal multilayer thin film (Patent Literature 3), and the like are disclosed.